The present invention relates generally to thin films on moving or stationary surfaces and, in particular, to near real-time monitoring of such films on the printing plates of offset printing machines.
Offset printing uses ink and dampening solutions to differentiate between the printing and non-printing areas of the printing plate. The hydrophilic property of the non-image areas and the oleophilic property of the image areas are the fundamental prerequisites of the offset printing process. In order to produce consistently high quality output, it is necessary to adequately coat the imaging areas of the printing plate with ink and the non-imaging areas with dampening solution. A clean, sharp image is produced when the ratio of ink to dampening solution is able to clearly distinguish between the non-imaging and imaging areas.
In simple terms, a non-printing area carries a layer of dampening solution, whereas a printing area has a layer of photolithographic film to which ink will adhere. There also will be a small contribution by dampening solution to the ink in a printing area.
In the art it is common to refer to dampening solution simply as xe2x80x9cwaterxe2x80x9d, leading to the expression xe2x80x9cink/water balancexe2x80x9d, referring to the ratio of ink to dampening solution, (or vice versa) between printing and non-printing areas.
If too little ink is applied the image becomes faint, colour balance incorrect, image density becomes non-uniform and sharp edges become blurred. On the other hand, if too much ink is applied, the image has poor edge definition, mottling of non-printing areas (scumming) can occur, incorrect colour balance results and there is an increased risk of ink rub-off and set-off (ie. transfer of ink between pages when folded).
Both over- and under-inking lead to a reduction in print quality and such printed material may have to be discarded. Maintaining the print quality above the minimum level leads to a reduction in wastage and saves paper, ink and dampening solution.
Different presses and operating conditions require different ink/water balances to achieve optimum print quality. In addition to achieving the correct balance between ink and water, the absolute amounts of ink and water have a secondary effect. Reduction of both water and ink will at some stage result in scumming and is to be avoided. Increase of both water and ink will maintain print quality, but will use wasteful amounts of both to achieve the same result. It will also result in rub-off and set-off where the excess ink is easily removed in the course of normal handling. The target that produces high quality print and highly economical print output is a correct ink/water balance marginally above the scumming levels. In practice, this target zone tends to change throughout the print run as conditions gradually change as a result of such factors as press temperature and wear on the printing plates.
The printing industry relies heavily on subjective measurements to determine the quality of the output product. In particular, visual inspection is used to grade a printed product against an agreed set of industry standards. The printing machine operators then adjust the ratio of ink to dampening solution based on acquired personal experience. Using the current method of visual inspection on a sampled point, gradual or intermittent deterioration in the print quality may not be noticed by the printing machine operators. The quality of the printing also may vary from print run to print run, depending on the personnel involved.
Attempts to find solutions to the problem of dampening solution thickness can be noted from U.S. Pat. No. 5,646,738 (issued to Fukuoka et al., on Feb. 29, 2000), U.S. Pat. No. 6,029,577 (issued to Honkawa et al., on Jul. 8, 1997) and U.S. Pat. No. 4,737,035 (issued to Aoki et al., on Apr. 12, 1988). None of these prior art documents provide a near-real time determination of changes in thickness of dampening solution. Nor do any one of the above-mentioned documents provide high spatial resolution measurement (ie. less than 1 mm2 measurement area).
U.S. Pat. No. 5,341,734 (issued to Jeschke et al, on Aug. 30, 1994) describes the removal of some dampening solution (using application of energy) from a test pattern area on a plate. The test pattern, therefore, prints at a lower dampening than the rest of the plate and so will begin to scum before the rest of the plate. Scumming is measured in the test area as a migration of ink into the non-imaging area.
U.S. Pat. No. 5,551,342 (issued to Fuchs et al, on Sep. 3, 1996), U.S. Pat. No. 5,568,769 (issued to Leuerer, on Oct. 29, 1996) and U.S. Pat. No. 4,649,502 (issued to Keller et al, on Mar. 10, 1987) all describe methods of measuring the ink density on the printed pages at the point of exit from the press. Ink density measurements are compared with either a stored set point or a measurement from an original proof copy and a feedback signal used to correct the press settings.
When a reduction in ink density is measured (on plate, blanket or paper) none of the prior art disclosures is capable of determining whether it is a result of under-inking (ie. not enough ink) or over-dampening (ie. too much water giving rise to excessive emulsification).
It is an object of the invention to accurately, and in near-real time gain an indication of the thickness of dampening solution and/or ink, thereby overcoming or at least ameliorating one or more problems associated with the prior art. It is also desirable to provide for the automated monitoring of both ink and dampening solution, and their relative amounts, and further, to provide control mechanisms for printing presses based thereupon.
There is provided a method for determining changes in the thickness of dampening solution in non-imaging areas of offset printing plates, comprising the steps of:
directing a light source at an offset printing plate, the light source being of a known intensity;
detecting light specularly reflected from said plate and forming an intensity signal thereof; and
determining a relative change in said detected light intensity signal as an indicator of changes in the thickness of dampening solution.
There is further provided an instrument for determining changes in the thickness of dampening solution on offset printing plates, comprising:
a light source for directing light of a known intensity at an offset printing plate;
a light detector for receiving light specularly reflected from said plate and producing an intensity signal thereof; and
a data processor for determining a relative change in the reflected light intensity signal, changes in which being an indicator of changes in the thickness of dampening solution.
The relative change in the detected light intensity signal may be determined as a ratio of the source light intensity signal and the detected light intensity signal, or directly from the detected light intensity signal when the source light intensity is held constant.
There is yet further provided a method for determining changes in the image density on a moving offset printing plate, comprising the steps of:
directing a light source at said offset printing plate;
detecting light scattered from said plate and forming a signal representative of the intensity thereof;
data processing said scattered intensity signal to identify imaging areas; and
further data processing said imaging data to determine image density in each imaging area.
There is yet further provided an instrument for determining changes in the image density on a moving offset printing plate, comprising:
a light source for directing light at an offset printing plate;
a light detector for receiving light scattered from said plate and producing an intensity signal thereof; and
a data processor for identifying imaging areas from said scattered intensity signal, and for determining image density in each imaging area.
For the non-imaging area, the scattered intensity signal is ascribed to be zero density, which value is used as a reference to determine the image density. A threshold value level can be defined to discriminate between imaging and non-imaging areas by comparing the scattering intensity signal of thereto.
There is yet further provided a method for determining the thickness of dampening solution on a moving offset printing plate, comprising the steps of:
directing a light source at said offset printing plate;
detecting light scattered from said plate and forming an intensity signal thereof;
detecting light specularly reflected from said plate and forming an intensity signal thereof;
data processing said scattered intensity signal to identify non-imaging areas;
filtering said reflected intensity signal to accept only data corresponding to said identified non-imaging areas; and
determining dampening solution thickness from said filtered data.
There is yet further provided an instrument for determining the thickness of dampening solution on a moving offset printing plate, comprising:
a light source at an offset printing plate;
a light detector for receiving light scattered from said plate and forming an intensity signal thereof;
a light detector for receiving light specularly reflected from said plate and forming an intensity signal thereof; and
a data processor for processing said scattered intensity signal to identify non-imaging areas, filtering said reflected intensity signal to accept only data corresponding to non-imaging areas, and determining dampening solution thickness from said filtered data.
Filtering can be achieved by applying a spatial mapping of the printing plate to the reflected intensity signal to pass only data relating to non-imaging areas. A threshold value level can be defined to discriminate between imaging and non-imaging areas by comparing the scattering intensity signal thereto.
There is yet further provided a method for determining the occurrence of scumming in an offset printing press, comprising the steps of:
directing a light source at said offset printing plate;
detecting light scattered from said plate and forming an intensity signal thereof;
storing a record of one or more positions representing non-imaging areas; and
data processing said scattered intensity signal to identify the presence of ink at said positions.
There is yet further provided an instrument for determining the occurrence of scumming in an offset printing press, comprising:
a light source for directing light at an offset printing plate, the light source being of a known intensity;
a light detector for receiving light scattered from said plate and producing an intensity signal thereof; and
a data processor storing a record of one or more positions representing non-imaging areas, and processing said scattered intensity signal to identify the presence of ink at said positions.
The scattered intensity signal may be compared against a threshold value that represents the onset of scumming. The threshold value can be derived from data collected from a printing plate absent ink.
There is yet further provided an optical instrument for determining properties of the imaging and non-imaging areas of moving offset printing plate, comprising:
a light source arranged to direct a beam of light towards said offset printing plate;
a first light detector arranged to receive light specularly reflected from said plate and produce an intensity signal thereof; and
a second light detector for detecting light scattered from said plate and produce an intensity signal thereof.
The light source may be arranged to direct light at an acute angle with respect to the printing plate surface. Furthermore, the first detector may be arranged at a similar acute angel, and the second detector arranged to be substantially normal to the printing plate surface. Most preferably, the light source is a laser source. The light source, first and second detectors may be housed within a single enclosure. There may be further provided a data processor for receiving and processing the reflected intensity signal and the scattered intensity signal.